This invention relates to a device for the feeding of flat materials for processing, such as sheets and envelopes, for machines using these materials, such as printers, comprising a frame and a movable support upon which a stack of said materials may be arranged and from which these materials are extracted one by one during selection cycles by selecting means engaging with a feeding mechanism and suitable for displacing the top material of the stack for feeding it into the machine, elastic means being provided for urging the movable support in the direction of the selecting means.
Devices such as these are known wherein the top sheet selection mechanism, upon each selection, requires great movements of parts, the full stroke of which is in relation to the maximum thickness of the stack of sheets that the movable support can accommodate. These great movements of parts, and of the movable support in particular, considerably slow down operation of the feeding device and cause annoying noises and rapid wear.
Another drawback of the known devices lies in the fact that the sheet selected is still being braked by the selection mechanism when it has already been entered in the printer and is being fed by the latter. As a result, there are variations of the paper feed pitch, giving rise to clearly visible defects, especially in high resolution graphics printing.
The object of this invention is to overcome the drawbacks outlined above; to this end, the invention is characterized in that the device comprises a retracting mechanism adapted in such a way as to push, during at least one phase of a selection cycle, the movable support back against the action of the elastic means by a predetermined distance which is substantially constant and independent of the thickness of the stack of materials, in order to produce a predetermined gap between the top material and the selecting means and of a magnitude such that friction between the top material and the selecting means is eliminated.
Thanks to these characteristics, the movements of the parts, and in particular those of the movable support for the sheets, may be reduced to a minimum during the selection, thus ensuring rapid, precise and silent operation, while lowering wear of the parts to a minimum. With the top sheet being distanced from the selecting means in a precise and controlled way, printing of great precision is obtained even in high resolution graphic printing.
To advantage, the feeding mechanism is adapted in such a way as to actuate both the selecting means and the retracting mechanism. Operation is particularly sure, while the construction remains simple and economical.
According to a preferred embodiment, the retracting mechanism comprises kinematic linkage members linking the driving mechanism with the movable support in such a way that the latter, in said phase, effects a predetermined movement starting from a first position wherein said stack is resting against the selecting means to a second position wherein the top material is removed from the selecting means by said predetermined gap, said movement being substantially constant whatever the position occupied by the movable support member in said first position and whatever the thickness of said stack. These characteristics guarantee great operating precision.
Advantageously the linkage members comprise a ratchet member cooperating actively with an opposing member following a first direction to displace the support member from the first position to the second position and permitting a free relative displacement of the ratchet member in relation to the opposing member following a second direction, opposite the first direction.
In a favourable embodiment, the opposing member is a ratchet wheel coupled in movement with the movable support and the ratchet mechanism is a pawl borne by a part, the displacement of which is controlled by the driving mechanism.
Thanks to these characteristics, the mode of operation is very safe and the number of parts limited. Said part is to advantage a connecting rod adapted for cooperating with a cam put in action by the driving means through the action of a clutching member. Separation of the top sheet from the selecting means can thus be controlled precisely and reliably.
In the known devices, it has often been remarked that the sheet feeding pitch in high resolution printing mode is not absolutely constant, which gives rise to images of poor quality. This drawback was to be ascribed to the fact that the sheet was braked to a varying extent in the guiding channel by the selecting means. In order to overcome this significant drawback, the device according to this invention comprises an actuating mechanism intended to broaden and narrow at least a part of a guiding channel for said materials during a phase of each selection cycle.
Accordingly when the sheet is being printed, the guiding channel may be broadened so that the friction forces are very low during the printing and a highly regular feeding of the sheet may be guaranteed. On the other hand, while the sheet is being fed towards the printing zone the guiding channel is narrowed to permit a very effective guiding of this sheet.
Other advantages will become apparent from the characteristics set down in the adjoining claims and from the description provided below explaining the invention in greater detail with the aid of drawings which represent, schematically and by way of example, one embodiment and variants thereof.